A number of types of EM digitizers or digitizer tablets are known in the art. In general, a digitizer tablet provides an indication of a position of a probe with respect to a surface of a planar sensor grid One type of digitizer tablet operates by sensing a magnetic field and includes two sets of array conductors, with one set being orthogonally oriented to the other set. In this type of system the probe is driven with an AC signal to cause an oscillating magnetic field which induces signals in the array conductors. The signals in the array conductors are sensed and compared so as to determine the position of the probe in two dimensions, relative to the surface of the tablet.
Other types of digitizer tablets are also known, such as tablets that drive the array conductors and sense the field with the probe. Tablets that operate with electric fields and resistive bridges are also known. Tablets that include magnetostrictive elements are also well represented in the prior art.
The magnetic field sensing tablet is of particular interest herein. More specifically, this type of tablet measures a mutual inductance of a pair of coils. One of the pair of coils is located within the tablet and the other pair is located within the probe, also referred to herein as a pen. The pen may be physically coupled to the tablet through wiring, referred to as a "tethered" pen, or may be physically decoupled from the tablet as an "untethered" pen. The untethered pen embodiment is preferred in that a more natural user interface is provided. However, the use of the untethered pen significantly complicates the exchange of signals between the pen and the tablet, such as pen state information generated by a contact switch coupled to the tip of the pen.
A related problem concerns the encoding of the pen state information. In that the pen position is estimated from measurements of the RF field generated by the pen coil, the pen state information encoded on the RF field may potentially distort the position estimate. For example, if a significant change in the pen signal amplitude is used to indicate a pen-down state, the amplitude change may directly effect the signal measurements, requiring that a position estimate in progress be discarded whenever the pen-down state changes. Such discarding of data points significantly degrades the performance of the system near pen-up and pen-down events.
The following chronologically ordered U.S. patents are representative of a significant number of U.S. patents that have been issued in this technical area.
In U.S. Pat. No. 3,626,483, issued Dec. 7, 1971, entitled "Spark Pen" to Whetstone et al., there is disclosed a writing stylus that produces a fast rise time shock energy sound wave, generated by a spark, that is detectable by microphones.
In U.S. Pat. No. 4,368,351, issued Jan. 11, 1983, entitled "Amplitude Modulated Digitizer" to Zimmer, there is described a digitizer having a pointer driven by an alternating carrier signal A grid of sequentially enabled parallel spaced conductors is inductively coupled to the pointer and includes an AM demodulator for detecting an instantaneous time of change of phase of the signal induced in the grid.
In U.S. Pat. No. 4,672,154, issued Jun. 9, 1987, entitled "Low Power, High Resolution Digitizing System with Cordless Pen/Mouse" to Rodgers et al., there is described a cordless pen that emits a directional electrical field The pen tip is capacitvely coupled to embedded conductors in a digitizer tablet. The frequency of the pen signal is modulated by several pen switches. Circuitry in the digitizing tablet discriminates the pen frequency to decode commands represented by switch closures between data scanning operations.
In U.S. Pat. No. 4,786,765, issued Nov. 22, 1988, entitled "Coordinates Input System" to Yamanami et al., there is described a system having an antenna coil 13 within a tablet body 12. The tablet body is driven by a position detection circuit. When an input pen 2 approaches the tablet a magnetostrictive vibration is enhanced by means of a bar magnet. A 455 kHz pulse signal is supplied from a NAND gate to the antenna coil 13 and is transmitted as radio waves. A tuning circuit 22a within the pen resonates with the transmitted radio waves. The tuning circuit 22a continues to resonate with decreasing amplitude after the transmission is stopped and generates a signal C, as shown in FIG. 8. The signal C is transmitted as radio waves by a coil 222 and is received by the tablet antenna coil 13. As such, this system shows a pen that is stimulated by a pulsed transmission from the tablet to generate a gradually attenuated RF signal after the tablet transmission ceases.
In U.S. Pat. No. 4,902,858, issued Feb. 20, 1990, entitled "Coordinates Input Apparatus" to Yamanami et al., there is described a system said to be capable of differentiating between types of position designating devices and capable of detecting factors concerning the position designating devices, such as the position and status thereof. As in the immediately above mentioned U.S. patent there is provided a tablet body 12 and an antenna coil 13 for use with a pen 2. An oscillator 401 sequentially generates a plurality of groups of clock pulses each comprising a predetermined number of clock pulses having different frequencies. A pulse signal having frequencies of 450 KHz, 500 KHz and 550 KHz is applied to NAND gates to become a signal B. The signal B has pulses of different pulse duration, depending upon which frequency signal is being transmitted. The signal B is sent to the antenna coil 13 for transmission to the input pen 2. The input pen 2 resonates with radio waves having one of the transmitted frequencies. As in the previous patent, the tuning circuit 22 continues to resonate after the transmission from the tablet ceases and generates a gradually attenuated signal C that is transmitted as radio waves for reception by the antenna coil 13. The use of pulses of different frequency is to differentiate between different types of position designating devices.
Finally, in German Patent No. DT 2650127, issued May, 1978, entitled "Pen Type Transducer Converting Hand Written Data to Coded Signals--Uses HV Pulses Applied to Pen and Has Printed Circuit Matrix" there is described a pen 10 that carries a high voltage pulse. The pen includes a high resistance and is used to write information on a paper sheet. Arrays of X, Y bistable circuits detect the position of the pen.
What is not taught by this prior art, and what is thus an object of the invention to provide, is a transmitting pen or stylus that autonomously generates pulses of a frequency signal for reception by a digitizer tablet.
A further object of the invention is to provide an inductive pen having a pulsed transmission characteristic for accommodating a modulation scheme that does not adversely impact pen position measurements, and that reduces tablet power consumption and amplifier gain requirements while improving noise immunity.